... right. So 10% of one month is about 1,000, so she has 20,000 to work with. She doesn't like the sound of selling shares of her basic income — that sounds like credit cards. And the ... account officer? ... said that she would be able to live alright on her basic income, meaning that her monthly expenses for a house and food and so on are probably about 10,000. So really she has 10,000 to work with, and needs to figure out whether to spend 2,000 or 4,000 on a phone.
She wasn't expecting to need a phone, but in hindsight it's kinda obvious that something like that would be required. Moreover, she's going to need more school supplies — paper and pencil, and maybe a skirt. But she's pretty sure those cost a lot less than a phone on Earth.
And she doesn't know how much of her basic income she will be able to save each month. Either she'll be able to save a lot, and get a new phone if she wants one in a few months, or she won't be able to save much, and she should hold onto the savings she does have. So either way, she should buy a cheap phone.
She enters the store and starts looking for a phone that fits comfortably in her hand, and is on the lower end of the 2,000-to-4,000 range.